1. Field of Invention
The present invention relates to setting and recovering of offshore marine structures by allowing buoyancy tanks to be placed at the bottom of marine structures and than pulling said structures on or off, of the deck of a barge thereby eliminating the need to use a barge and expensive derrick barge to move the structure into or out of the water. The present invention has been found to be particularly useful in the offshore construction and salvage art, and hence will be discussed with particular reference thereto. However, the present invention is applicable to other types of construction requiring moving, setting, salvaging and demolition of offshore structures.
2. Description of Prior Art
A derrick barge or a crane ship is a very useful and well used tool in the offshore industry for assisting in the construction of offshore structures. However, their lifting capacity is limited by weight lifting capacity, by the height of the structure being built and by the cost of the derrick barge or crane ship. To date the largest crane ship can lift about 4,500 short tons and is very expensive to operate.
To operate a crane ship, or derrick barge a deck cargo barge is also required to transport the jacket structure. This will also usually require several offshore tug boats.
To increase the capacity of a derrick barge or a crane ship the boom must be made stronger, the cables must be made of a larger diameter, the number of cables must also be increased and the winch must be made stronger. With the increased capacity of the boom, cables and winch a lot of structural steel must also be added to the crane and the vessel. The extra steel, cables, machinery etc., are added along the water line of the vessel thereby decreasing the stability of the vessel. When the stability of the vessel is decreased, the vessel itself must be enlarged. As the weight lifting capacity of a derrick barge or a crane ship is increased, the cost of the unit will increase exponentially.
As the operating criteria of production platforms is increased, so is the height and weight of the production platform increased. As the height and weight of the platform are increased the reach and weight lifting capacity of a derrick barge or crane ship must be increased. A derrick barge or crane vessel must reach out to the center and above the item to be lifted and is therefore at a great mechanical disadvantage when working either well below or a great distance away from the center of the platform.
Another method of handling offshore jacket structures is to use buoyant tanks that are attached to the sides of the structure by divers. This allows the structure to be removed from the operation site but the structure must be transported below the water with only the buoyant tanks at the surface. The problem with this method of jacket structure recovery is that the jacket structure is deep in the water and therefore cannot be taken into a harbor or port to be worked on. It is also good only for jacket structure salvage.
Several types of offshore platform construction method have been known and used before and typical examples thereof are shown in U.S. Pat. No. 3,442,340 May 6, 1969 issued to Lowell B. Christenson, U.S. Pat. No. 2,907,172 issued to Wilfred S. Crake Oct. 6, 1959; U.S. Pat. No. 4,041,711 issued Aug. 16, 1967 to Joseph E. Lucas. U.S. Pat. No. 3,138,932 June 30, 1964 issued to Darld C. Kofahl. None of these devices, however, teach either setting or salvaging offshore jacket structures that are widely used in the offshore industry.